# Quantum Communication

### Description

As the field of quantum communication continues to evolve and advance, there is a growing need for professionals with the knowledge and skills to tackle the complex challenges and opportunities it presents.

The course covers the fundamental concepts of quantum communication, including the properties of qubits and quantum registers, the manipulation of quantum states through quantum gates, and the principles and implementations of quantum key distribution.

Additionally, the course delves into various quantum communication protocols and their respective approaches, and the security challenges in quantum communication, such as attacks against quantum key distribution protocols, and the potential applications and future developments of a quantum internet.

### Participants attending this course will

- Be able to apply their knowledge of quantum key distribution
- Be equipped to utilize key distribution protocols
- Be able to comply with existing quantum communication standards

### Outline

- Introduction to quantum communication
- Qubit and quantum register
- Quantum gates
- Quantum key distribution
- Quantum communication protocols
- Quantum key distribution approaches
- Attacks against QKD protocols
- Quantum internet

### Course information

#### Preparedness

Fundamental knowledge of quantum computing

#### Exercises

Hands-on

#### Delivery methods

Onsite / Virtual classroom

### Related courses

### Table of contents

- Day 1
- Day 2
- Day 3

- What is quantum communication about?
- Qubit and quantum register
- Four postulates
- Quantum bits
- 1st postulate in details - qubit
- Quantum bit (qubit)
- Quantum bit with real probability amplitudes
- Important qubits
- Qubits in practice

- Quantum registers (quregisters)
- 4th postulate in details - quantum register
- What is a tensor product?
- Matrix Multiplication
- Matrix Exponentiation
- How to calculate square of matrix A
- Transponent of a matrix
- Tensor product in practice - example
- Quantum registers

- Quantum gates
- 2nd postulate in details
- Unitary transformation
- Sidenote: mathematical background
- Sidenote: mathematical beckground - inner and outer product
- How does a quantum gate look like?
- One Qubit gates
- Identity gate
- Pauli X gate, or bit-flip gate
- Pauli Z gate, or phase-flip gate
- Pauli Y gate
- Pauli gates and the Bloch sphere
- Phase rotator gate
- Hadamard gate

- Two (or more) Qubits gates
- n-qubit Hadamard gate
- Controlled NOT gate (CNOT gate)
- Controlled Z gate (CZ gate)
- SWAP gate
- Toffoli gate ("controlled-controlled-not" gate)
- Toffoli gate and Hadamard gate
- Fredkin gate
- Bloch Sphere Simulator

- Extracting information from quantum registers (Measurements)
- 3rd postulate in details
- 3rd postulate using ket notations
- Projective measurement
- How to calculate measurement operators?
- How to write the measurement operators?
- Completeness relation
- Projective measurement - practical notation
- 3rd postulate in case of projective measurement
- How measurement works?
- Measurement using computational basis states
- Repeated projective measurement
- What is randomness?
- How to create projective measurement?

- Quantum bits

- Four postulates
- IBM Quantum
- How to prepare a superposition?
- Preparing an arbitrary quantum state

- No cloning theorem
- No Cloning Theorem - Proof

- Entanglement
- Decomposition exercise
- Entangled states
- Difference between product and entangled states
- What does entanglement mean?
- Famous entanglement pairs
- How to produce entangled pairs?
- Changing the bases of an entangled pair
- CNOT gate
- Bell state generator
- Generalized quantum entangler
- How to create entangled qubits physically? - An example

- Implementation examples for qubits
- Physical qubits
- Di Vincenzo criteria
- Superconducting qubits
- Trapped ions
- Photonic qubits

- Quantum key distribution over optical fiber
- From theory to the real world
- Protocol stack of QKD
- Classical post processing
- Information reconciliation
- Privacy amplification
- Missing piece

- Two important quantum communication protocols
- Quantum Communication – the players
- Superdense coding
- Bell state measurement

- Teleportation
- Teleportation – Option 1
- Teleportation – Option 2
- Teleportation – remarks

- Superdense coding and Teleportation

- Quantum Medium Access Control
- Medium Access Control in distributed environment - quantum WIFI
- Quantum WIFI - MAC
- Quantum MAC - JOIN
- Quantum MAC - LEAVE
- Quantum MAC - TRANSFER
- Quantum MAC - generalization
- Rebalacing
- Quantum periodic table

- Density matrix and its application in quantum communications
- Decomposition exercise
- Introduction to density matrix formalism
- Calculation of density matrices
- Mixed states
- Geometrical representation
- Are density matrices unique?
- Partial trace
- Postulates with density matrices
- Teleportation
- State vs. density matrix formalisms

- Why do we need quantum key distribution?
- Problem formulation
- Symmetric Keys
- Symmetric Keys – problems and solutions
- Asymmetric Key System
- Problem formulation
- Problem formulation – Quantum Key Distribution

- Quantum key distribution approaches
- Two types of QKD
- Prepare-and-measure QKD
- Entanglement based QKD

- In real life, quantum channel is not ideal
- BB84, the first QKD protocol
- BB84 in a nutshell
- BB84
- Without an attacker
- With an attacker
- Flowchart of the BB84 protocol
- Summary of BB84

- SARG04 protocols
- SARG04

- B92 protocol
- Entanglement based QKD
- 2- and 3-party BB84
- Entanglement based protocol

- E91 protocol

- Two types of QKD
- Passive and active attacks
- Passive attack
- Active attack
- Attack types
- Denial of service attack
- Man-in-the-middle attack

- Non idealistic channels
- Why realistic quantum channels are important?
- General model of communication
- AWGN
- Channel capacity
- A suprising example
- Various quantum capacities
- Basic quantum channel models
- Effects in real fibers

- Interesting QKD approaches
- Chicago Quantum Exchange
- Chinese Quantum Network: More than 4600 kilometers
- UK Quantum Communications Hub
- EuroQCI
- Companies: ID Quantique
- Companies: MagiQ
- Companies: Quintessence Labs

- Technical study: implementing BB84
- High level overview
- Physical realization

- Quantum key distribution via free-space
- Why to use free-space instead of fiber-based solution?

- Errors in free-space quantum channels
- Quantum signals on free-space channels
- There are equations behind everything
- But this is what is important

- Prepare-and-measure QKD vs entanglement-based QKD
- Prepare-and-measure QKD -advantages
- Prepare-and-measure QKD -disadvantages
- Entanglement-based QKD -advantages
- Entanglement-based QKD -disadvantages

- Physical attacks on quantum key distribution protocols
- Intercept and resend
- Faked states attack
- Photon number splitting attack
- Trojan Horse Attack (Light Injection Attack)
- Using a beam splitter

- Interesting free-space QKD approaches
- Scenarios for satellite based quantum communicatoins
- How to use an entanglement-based satellite system?
- Quantum Experiments at Space Scale (QUESS)
- Singapore’s mission in 2019
- SpooQy 1
- European answer: SAGA
- A draft concept of the European SAGA system
- Timeline for the SpaceQCI (from European Comission)
- Timeline for the SAGA mission (from ESA)
- Security threats to a satellite QKD system

- Quantum Secure Network, Quantum Information Network
- Stages to achieve a global quantum internet

- Ingredients of future's quantum internet
- Challenges
- Entanglement swapping
- Entanglement swapping - remarks
- Routing

- Overview of standardization
- Working on standards
- Working on quantum communications standards
- ETSI Quantum-Safe Cryptography (QSC) working group
- ETSI - Quantum-Safe Cryptography (QSC)
- ETSI Industry Specification Group (ISG)
- ETSI QKD
- ETSI Standards – An example
- IETF QIRG – Standards for quantum internet
- IETF QIRG – Two dradts

- Quantum communication: implementation challenges
- Technical study: implementing CV-QKD system
- A Hungarian CV-QKD system

- Technical study: implementing quantum random number generator (QRNG)
- The simplest QRNG
- A Hungarian QRNG system

- Summary and outlook
- Future's satellite system
- QKD and autonomous vehicles
- QKD and 5G